In monitoring the condition of a patient's heart, potentials produced by heart action at different points on the body are picked up by electrodes applied to these points and the differences between the potentials are coupled to the monitoring apparatus that is referenced to true ground via suitable circuits. In order to protect the patient from the possibility of electrical shock, any path between the patient and true ground must have a very high impedance. Accordingly, any circuit that is directly coupled to the patient is referenced to a floating ground called a "guard". Unfortunately, however, the patient is generally within one or more ambient electrical fields from such sources as lights or power cords that produce what is known as a "common mode voltage", V.sub.CM, on his body. The impedance looking back from each electrode to the patient's body and the impedance looking forward from each electrode to the floating ground form a conventional four-element bridge circuit which is excited by some fraction of the V.sub.CM. If the bridge happens to be in balance, the V.sub.CM introduces no problem, but this is seldom if ever the case because the impedance between each electrode and the patient's body can vary over wide limits. Any imbalance causes a portion of the common mode potential V.sub.CM to add to or subtract from the difference between the potentials at the points of interest on the body so as to cause errors in the signal derived therefrom.
A solution to this problem that has been used for a long time is to apply a reference electrode to the patient's body and connect it to the floating ground or guard in such manner as to make the patient have nearly the same potential as the guard, thereby reducing the effect of the common mode voltage on the floating circuit.
Whereas this scheme works well, the reference electrode is a source of error if it is not properly applied, so that as much care and time must be taken in applying it to the patient's body as in applying the other electrodes. Furthermore, because it provides no useful physiological information it may be a source of confusion to a user.
In his U.S. Patent Application filed concurrently herewith and entitled "Coupling Circuit With Driven Guard", Timothy B. Blancke has described an invention that eliminates the need for a reference electrode without in any way impairing the safety of the patient. At the same time, excellent rejection of the effects of common mode potentials is attained. The circuits connected to the patient are referred to floating ground or guard as before, but instead of driving the potential of the patient toward the common mode potential of the guard, as has been done, the guard is driven toward the common mode potential of the patient. This is accomplished by applying the common mode potential on the floating circuits to control means for causing current to flow from true ground through the stray impedance between guard and true ground. If the current has the correct value and direction, the guard will have the same common mode potential as the floating circuits. Under such conditions, the bridge is not excited by the common mode potential and no addition to or subtraction from the desired signal voltages occurs.
In order to protect the patient from electrical shock, the circuits shown in the application referred to utilize current limiters. Such devices are expensive and subject to failure.